1. Field of the Invention
The present invention relates in general to a method of refining tungsten hexafluoride, and more particularly to a method of removing molybdenum hexafluoride as an impurity from tungsten hexafluoride.
2. Description of the Prior Art
There is an increasing demand for tungsten hexafluoride of very high purity. This type of tungsten hexafluoride is used, for example, as a material for sputtering target, conductive paste or semiconductor electrode.
According to one of conventional methods, tungsten hexafluoride is produced by the reaction of tungsten powder with fluorine gas. This tungsten powder is prepared by a so-called wet method in which tungsten oxide produced by hydrolysis of ammonium paratungstate is reduced to tungsten powder in the atmosphere of hydrogen. To produce tungsten hexafluoride of very high purity, it is necessary to prepare tungsten powder of very high purity. However, tungsten powder is inevitably somewhat contaminated with impurities derived from raw materials and/or reagents. Therefore, tungsten hexafluoride produced from tungsten powder is also inevitably contaminated with the impurities. Thus, it is necessary to refine tungsten hexafluoride, for example, through distillation. With this, most of impurities can be removed from tungsten hexafluoride by the difference therebetween in boiling point or vapor pressure. However, molybdenum which is one of the impurities of tungsten hexafluoride has melting and boiling points close to those of tungsten hexafluoride. In fact, melting points of tungsten hexafluoride and molybdenum hexafluoride are 2.degree. C. and 17.5.degree. C., and boiling points of those are 17.2.degree. C. and 35.degree. C., respectively. Furthermore, molybdenum hexafluoride has a very high vapor pressure even at a temperature under melting point thereof. Therefore, it is practically impossible to remove molybdenum hexafluoride from tungsten hexafluoride through distillation. Thus, it is usual that tungsten hexafluoride refined by a conventional manner contains a few ppm to several hundreds ppm of molybdenum hexafluoride. This contamination affects an electrical characteristic of tungsten hexafluoride, thereby deteriorating quality thereof as a final product.